Transport and storage of potassium in the Earth's upper mantle and transition zone: an experimental study to 23 GPa in simplified and natural bulk compositions

We have investigated the stability and composition of Potassium amphibole and its high-pressure breakdown product phase X in synthetic peralkaline and subalkaline KNCMASH (K(2)O-Na(2)O-CaO-MgO-Al(2)O(3)-SiO(2)-H(2)O) and natural KLB-1 pen dotite bulk compositions between 10 and 23 GPa at 800-1800 degrees C. In the KNCMASH system, potassium amphibole reaches its upper pressure stability limit at 13-15 GPa at less than or equal to 1400 degrees C. In the natural KLB-1 bull; composition, potassium amphibole breaks down between 12 and 13 GPa at 1200 degrees C. Phase X is a hydrous potassium-magnesium silicate with variable stoichiometry: a general formula K(2-x)Mg(2)Si(2)O(7)H(x) with x = 0-1, and a maximum possible H(2)O content of 3.5 wt %. Electron microprobe analytical totals suggest H(2)O contents of similar to 1-2 wt % and a decrease in H(2)O contents with increasing pressure. In both KNCMASH and KLB-1 systems, phase X coexists with Mg(2)SiO(4) + garnet + high-Ca clinopyraxene + low-Ca clinopyroxene +/- fluid. Phase X breaks down between 20 and 23 GPa at 1500-1700 degrees C to form K-hollandite + gamma-Mg(2)SiO(4) + marjorite + Ca-perovskite + fluid. The upper temperature stability limit of phase X was located in the subalkaline KNCMASH system between 1400 and 1600 degrees C at 14 GPa and at >1700 degrees C at 20 GPa, the latter being at least 200 degrees C above an average current mantle adiabat. Thus, phase X could store and transport both water and potassium not only in subduction zone settings, but also in convecting mantle down to the transition zone-lower-mantle boundary. Phase X would also be an eminently suitable host for Rb, Cs, Ba or Pb.